VITT anti-PF4 reactivity to PF4/polyanion complexes and uncomplexed PF4 were stable after BNT162b2 booster vaccination. A Food and Drug Administration–approved in vitro diagnostic assay, LIFECODES PF4 IgG (Immucor), was used to assess VITT antibody reactivity (A) and titer (B) toward PF4/polyanion complexes prior to and after administration (vertical dashed line) of mRNA BNT162b2 vaccine booster. Similarly, a novel ELISA employing uncomplexed PF4 targets was used to define VITT antibody reactivity (C) and titer (D) to PF4 not complexed to polyanions. Time in days relative to booster vaccination is shown on the x-axes of panels A and C, and the level of sample dilution is shown on the x-axes of panels B and D.